1. Field of the Invention
This invention relates to a glass material suitable for a substrate for information recording media such as magnetic discs, optical discs, and the like. More particularly, this invention relates to a substrate material for information recording media such as magnetic recording medium discs made of a glass material having excellent flatness and strength, as well as high heat resistance, and being durable against a higher heat treatment in order for a coercive force improvement of a magnetic layer, a substrate using this material, an information recording medium using this substrate, and a manufacturing method for such information recording media.
2. Description of Related Art
As a glass substrate for, e.g., a magnetic disc (hard disc), a substrate made of a chemically tempered glass whose substrate surface is reinforced by an ion exchange and a crystallized glass substrate whose substrate surface is subject to a crystallization treatment, have been known previously.
For example, as a chemically tempered glass substrate, Japanese Unexamined Patent Publication No. Hei 1-239,036 (hereinafter referred to as Reference 1) discloses a glass substrate for magnetic recording media in which glass containing, by weight percent, SiO.sub.2 of 60 to 70 percent, Al.sub.2 O.sub.3 of 0.5 to 14 percent, R.sub.2 O (R denotes alkali metal) of 10 to 32 percent, ZnO of 1 to 15 percent, and B.sub.2 O.sub.3 of 1.1 to 14 percent, is reinforced by an ion exchange.
As a crystallized glass, Japanese Unexamined Patent Publication No. Hei 7-187,711 (hereinafter referred to as Reference 2) discloses a glass substrate for magnetic recording media including kanasite as a main crystal and, by weight percent, SiO.sub.2 of 50 to 65 percent, CaO of 18 to 25 percent, Na.sub.2 O of 6 to 11 percent, K.sub.2 O of 6 to 12 percent, Al.sub.2 O.sub.3 of 0 to 2.5 percent, and F of 5 to 9 percent.
Moreover, Japanese Unexamined Patent Publication No. Hei 8-169,724 (hereinafter referred to as Reference 3) discloses, as a SiO.sub.2 --Al.sub.2 O.sub.3 --RO (R denotes bivalent metal) system glass, a high expansion, heat-resisting glass used for magnetic discs or the like having a transition temperature (Tg) of 650 degrees Celsius or greater and a thermal expansion coefficient (.alpha.) of a range of 80 to 110.times.10.sup.-7 /.degree.C. which contains, by weight percent of total amounts of inevitable components, SiO.sub.2 +Al.sub.2 O.sub.3 of 35 to 55 percent, and CaO+BaO of 40 to 60 percent.
Meanwhile, magnetic recording media have been made year after year with higher and higher recording densities, and therefore, there are various demands for properties of glass substrates for magnetic recording media. One of such demands is high heat resistance. Magnetic characteristics of a magnetic layer (magnetic recording layer) such as coercive force have to be improved to increase the recording density of the magnetic recording media. The coercive force of the magnetic layer, though may vary on magnetic materials to be used, can be made higher by a heat treatment even if made of the same materials. Therefore, in addition to developments for new magnetic materials, it has been expected to thermally treat a magnetic layer formed on a substrate at a higher temperature for the purpose of obtaining higher coercive force using currently existing materials.
The conventional chemically tempered glass as disclosed in Reference 1, however, has a glass transition temperature around 500 degrees Celsius. To the contrary, to improve the magnetic characteristics such as coercive force of the magnetic layer, the heat treatment is effective when implemented at a temperature higher than 500 degrees Celsius. Accordingly, the chemically tempered glass as described in Reference 1 itself does not have enough heat resistance. Such chemically tempered glasses generally have an ion exchange layer for alkali metal ions on the surface of the glasses. When a magnetic layer is formed on the surface of the chemically tempered glass and subject to a heat treatment, however, there arises a problem that the ions in the ion exchange layer immigrate in the magnetic layer and affect the magnetic layer unfavorably. A higher temperature will further activate the immigrations of the alkali metal ions into the magnetic layer. To suppress such alkali ion's immigrations, it is favorable to perform a heat treatment at a further lower temperature. Thus, magnetic characteristics may rarely be improved where the chemically tempered glass substrate is used and subject to a heat treatment at a high temperature, and it is difficult to obtain magnetic recording media having a high coercive force.
Another conventional crystallized glass as disclosed in Reference 2 has an excellent heat resistance because the glass does not show transition. However, the glass substrate for magnetic recording media requires more surface smoothness as the recording density increases. This is because high density recording in the magnetic recording media requires a lower frying of the magnetic head. Since the crystallized glass contains many micro-particles, it is impossible to form substrates with a surface roughness (Ra) of 10 angstroms or less. Consequently, the substrates likely have poor surface flatness and impaired surface form. A roughness control layer, for example, may be formed on a substrate to prevent the magnetic head from clinging to the magnetic disc. However, there arises a problem that on the substrate made of the crystallized glass, it is difficult to control surface morphology of the roughness control layer.
The glass described in Reference 3 has a transition temperature of at most 730 degrees Celsius. Glass substrates generally require a treatment implemented at a temperature of about 40 or more degree Celsius below the transition temperature, to be subject to the heat treatment without deformation of the glass substrates. A glass having a transition temperature of 730 degrees Celsius should has to endure a heat treatment implemented at most at 680 degrees Celsius. However, a next generation magnetic recording media require to meet further stricter demands for magnetic characteristics, and as a result, the media needs a heat treatment implemented at a further higher temperature, e.g., a temperature exceeding 700 degrees Celsius. The glass disclosed in Reference 3 cannot endure such a high heat treatment.
A carbon substrate for magnetic recording media has been known as having good heat resistance, as disclosed in, e.g., Japanese Unexamined Patent Publication No. Hei 3-273,525. The carbon substrate, however, has a mechanical strength inferior to glass and may hardly correspond to a thinner substrate trend that is demanded as magnetic discs are made smaller.
This invention is devised in consideration to the above problems. It is a first object of the invention to provide a material suitable for a substrate for information recording media made of glass having excellent surface flatness and mechanical strength, the material having excellent characteristics durable against a heat treatment at a higher temperature for improving magnetic characteristics of a magnetic layer, and to provide a substrate using this material.
It is a second object of the invention to provide a magnetic recording medium capable of lowering a frying height of a magnetic head, increasing recording density, making thinner, or more compact, the substrate, and improving the magnetic characteristics in the best fashion, and to provide a manufacturing method for the magnetic recording medium.